Steam generators, particularly those of the coal fired type, can generate harmful emissions. Recent efforts have focused on oxygen firing (e.g., oxy-combustion) which injects oxygen into a duct that transports flue gas into a fuel supply system (e.g., coal pulverizer system) and/or the steam generator. The oxygen can be supplied from an air separation unit. Due to the elimination of the inherent nitrogen that occurs with air firing, oxygen firing results in an essentially pure carbon dioxide product gas which can be more efficiently sequestered. Most oxygen fired steam generators utilize significant flue gas recirculation in order to moderate furnace temperatures. Gas recirculation at high rates adds considerable cost, complexity, and increases the need for auxiliary power.
Typically oxygen from the air separation unit is mixed with the recycled flue gas streams forming an oxidant stream before entering the steam generator. To match air-firing thermal performance a global oxygen concentration of approximately 28% is required. The oxygen concentration in this global oxidant is higher than typical air due to differences in gas density and heat capacities between air and recycled flue gas. There may be some limits placed on the concentration of oxygen in the oxidant stream. For example, the primary oxidant stream which transports pulverized fuel to the steam generator may be limited to about the 21% oxygen content of air to avoid problems with premature combustion of the fuel. Also oxygen contents much above 21% may require that ducts and other components be made of more expensive, higher grade materials suitable for the higher oxygen content. The duct and component limit is 23.5% for temperatures of 200-900° F., to which the oxidant typically is subject.